Recently, there has been interest in a new type of light emitting device called a vertical cavity surface emitting laser (VCSEL). Conventional VCSELs have several potential advantages, such as a planar construction, emitting light perpendicular to the surface of the die, and the possibility of array fabrication.
Typically, VCSELs include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
The use of mirror stacks in VCSELs is well established in the art. Typically, mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an Al.sub.x1 Ga.sub.1-x1 As.backslash.Al.sub.x2 Ga.sub.1-x2 As material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content x1 and x2 in the layers, more particularly the aluminum content x1 ranges from 0% to 50% and the aluminum content of x2 ranges from 50% to 100%. In conventional devices, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
One application for VCSELs that is becoming quite common is in the optical communication field where optical fibers are utilized. In utilizing optical fibers for communications, it is desired to have the highest possible coupling efficiency between the light source(s) and the fibers. In free space optical communications it is desired to collimate the beam of light emitted by the light source(s). When a conventional VCSEL is utilized as the light source, an external lens or lens array is used to either focus an emitted beam of light into the fibers, or collimate the beam into the receivers directly in the free space. This external lens or lens array is often quite bulky and does not lend itself to a small overall package size.
In an effort to reduce the size of the package and the cost of manufacture, is has been suggested to use the VCSEL substrate as a lens using photolithography etching methods. This technique works for lasers of 980 nm or longer in that when formed on a transparent GaAs substrate, the substrate does not absorb the laser emission, thus the light is emitted through the lens formed on the GaAs substrate. When a VCSEL of 850 nm or shorter is utilized, the VCSEL emits in an upward direction, opposite the substrate element, and does not pass therethrough the GaAs substrate. This upward emission is a requirement in that the GaAs substrate is absorptive to light in a wavelength of 850 nm or shorter.
Therefore, it can be readily be seen that a conventional VCSEL that has integrated as a part thereof a lens element formed on a substrate element cannot be utilized in a VCSEL that emits light of 850 nm or less. Thus, there is a need for developing a reliable, stable and cost effective vertical cavity surface emitting laser (VCSEL), that includes an integrated lens element for VCSELs that emit light of a short wavelength, more particularly light having a wavelength of 880 nm or less.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. Accordingly, it is an object of the present invention to provide a new and improved VCSEL with integrated lens element for use in short wavelength VCSEL applications.
Another object of the invention is to provide a reliable VCSEL with integrated lens element for optical communications applications.
And another object of the immediate invention is to provide for a VCSEL with integrated lens element that provides for beam focusing or beam collimation of short wavelength emitted light.
Yet another object of the invention is to provide for a highly manufacturable VCSEL with an integrated lens element.